Modelling the integrated strategies of deficit irrigation, nitrogen fertilization, and biochar addition for winter wheat by AquaCrop based on a two-year field study

Biochar application has been proposed as a promising practice for sustainability due to its potential to ensure food security while alleviating the water shortage and fertilizer overuse. Scheduling the proper integrated management of deficit irrigation, nitrogen fertilization, and biochar addition are necessary to facilitate the effective use of biochar in agriculture. The objectives of this study were to test the AquaCrop model for winter wheat under different irrigation, nitrogen, and biochar regimes and to optimize the integrated strategies. For the model calibration and validation, a field experiment of winter wheat with two irrigation levels (full and deficit irrigation) and three fertility levels (200 kg/ha N, 100 kg N + 15 t/ha biochar, 0 kg/ha N) was conducted in the North China Plain during 2017/2018 and 2018/2019 growing seasons. The results showed that AquaCrop performed well in simulating soil water content (0.82 ≤ R2, 0.92 ≤ d, RMSE ≤ 2.82%, NRMSE ≤ 17.34%), canopy cover (0.89 ≤ R2, 0.95 ≤ d, RMSE ≤ 7.58%, NRMSE ≤ 13.33%), biomass (0.96 ≤ R2, 0.96 ≤ d, RMSE ≤ 1.04 t/ha, NRMSE ≤ 16.75%), and grain yield (0.94 ≤ R2, 0.97 ≤ d, RMSE ≤ 0.34 t/ha, NRMSE ≤ 5.32%). The effects of biochar on soil moisture, fertility, and wheat production could be adequately simulated by AquaCrop using the hydraulic properties of the biochar-mixed soil layer and its semi-quantitative method for soil fertility stress. The optimal integrated strategies were deficit irrigation with three irrigations of 60 mm at the jointing, anthesis, and grain-filling stage for dry years, two irrigations of 60 mm at the jointing and anthesis stage for normal years, and one irrigation of 60 mm at the anthesis stage for wet years, combined with 100 kg/ha N and 15 t/ha biochar, which could achieve over 80% of the potential yield while substantially increasing irrigation water productivity. Nonetheless, the model tended to under-estimate soil moisture, leaf growth and senescence, and grain yield in the biochar amended treatments, particularly under drought. Further improvements in the modules of soil properties, soil water balance, and canopy cover can be performed to increase the estimation accuracy. Additionally, AquaCrop was only calibrated and validated based on a two-year experiment under the weather conditions of no drought and moderate drought. Therefore, future validation studies using the long-term field data with the larger climate variability are necessary to better identify the impacts of integrated strategies and climate change. Finally, considering its good balance in simplicity, robustness, and accuracy, AquaCrop could be a reliable tool to guide the field management of irrigation, fertilization, and biochar addition.